My thinking with the scales is that I'm not sure I'd expect Dm6 to
be present in the Dm scale?

But I'm not knowledgable on the subject, only
opionionated.

-n.

A 6th chord is an extended chord. It wouldn't be present from harmonising in triads, or
sevenths, the conventional ways, but that's the point of an extended chord, to added a
different flavour to a sound.

That
(DFAB) is right yes - it's a minor triad plus the sixth -the major sixth.

Adding the minor sixth is possible too, but discordant because the A and Bb are only a
semitone apart - but if you can lose the fifth or find a better inversion, it's not
unusable. I think you notate it explicitly as Dm6/min6 or similar.

A minor 6 chord always has the notes (from the root) 1-b3-5-6: adding Bb to a Dm triad
makes Dmb6, which as an earlier poster pointed out is a lot harder to voice nicely. You
need to either lose the 5, or make sure the 6 is a long way above it (which really makes a
Dm add b13 chord).

Do 6 chords change relative to the key? You could look at it
like that, but in practice, not really. The only minor 6 chord you can make diatonically
in a major key is the one built on the second degree: thus Dm6 lives in the key of C
major, not F major.

A common use for Dm6 is as a substitute for Bm7b5: in fact
it contains the same notes. Dm6 is the 1st inversion of Bm7b5 (it's built from the 3rd
instead of the root). In fact, if you take a look at the different 6 chords you can make
in a major key, they're all inversions of 7th chords (C6=Am7, Dm6=Bm7b5, Emb6=Cmaj7,
F6=Dm7, G6=Em7, Amb6=Fmaj7, Bmb6b5 (ugh)=G7).

Minor keys are another matter,
but remember the chords in D minor are not built from the same notes that are found in F
major (for example). If you do make a minor chord sequence from the diatonic notes of the
relative major (i.e. using the notes of C major to build an A minor sequence, or using F
major to do D minor), you're not actually in the key of A minor or whatever: you'd be in
the A or D Aeolian mode. Minor keys (usually) use the notes of the Harmonic Minor scale to
build chords, but because minor harmonies are very unstable there's a lot of leeway for
chromaticism and non-diatonic notes: in fact, although there's a theoretical 'right
answer', I'd say that in practice it's impossible to say exactly what harmonies are
diatonic to a minor key.

If
you do make a minor chord sequence from the diatonic notes of the relative major (i.e.
using the notes of C major to build an A minor sequence, or using F major to do D minor),
you're not actually in the key of A minor or whatever: you'd be in the A or D Aeolian
mode. Minor keys (usually) use the notes of the Harmonic Minor scale to build chords, but
because minor harmonies are very unstable there's a lot of leeway for chromaticism and
non-diatonic notes: in fact, although there's a theoretical 'right answer', I'd say that
in practice it's impossible to say exactly what harmonies are diatonic to a minor key.

Sorry that got so technical. Once you start me off...

This leads nicely to a question I've
always wanted to ask..but never got round to.

When did the Harmonic Minor
scale fall out of favour in Western music and why?

I've studied Bach at
school (G.C.S.E and A Level) and obviously Harmonic Minor was commonplace in the Baroque
period but you don't hear so much of it in very late Classical or Romantic music.

I read somewhere that melodies derived from the Harmonic Minor were difficult to
sing and the Melodic minor scales were devised for that reason.

Quote Rhys Llewellyn:A 6th chord
is an extended chord. It wouldn't be present from harmonising in triads, or sevenths, the
conventional ways, but that's the point of an extended chord, to added a different flavour
to a sound.

Small point of
pedantry (since pedantry is, after all, the entire raison d'etre of this board and the
reason we all love it so much) - Dm6 is
actually an Added Chord, meaning that the sixth is just superimposed upon the triad
"because it sounds good". "Extended chord" relates to chords that are built up by
thirds, but which have the more distant additions - 9ths, 11ths, 13ths.

Carlos
- You need to remember that there are two things at play, the technical derivation of all
the various chords and then the process by which people choose which of those chords to
use, according to what sounds stylistic and what works.

The chord symbol "Dm6"
always mean a D minor triad with a B natural added - ie, any "m6" chord will have a MAJOR
6th added above the root.

It's perfectly possible to add a minor 6th above the
root instead, and indeed that's what you'll get if you form chords in the same way on
various different scale degrees, as you've found. I'm not sure what the chord symbol would
be for such a chord, since no-one has every really bothered to give it one.

There's a reason for this, which goes beyond the idea that it just "doesn't sound good"
having the semitone between the fifthe and sixth (after all plenty of voicings of various
other chords have semitones in them, it's all a matter of context). The real issue is that
Dm with a Bb added is really just Bb Maj7 in first inversion. When you play it, that's
exactly what it sounds like - your ear immediately hears Bb as the root of the chord,
maybe just slightly weakened by there being a D in the bass.

So if you really
want it played with D in the bass, you'd probably write it "Bb Maj7 (D Bass)", or "Bb
Maj7/D".

"OK, so why can't Dm with a B natural added just equally be seen as a
B half diminished seventh in first inversion?" I hear the clamorous tones of a million
outraged musicologists cry...

It can, of course, but there's a big difference.
With the Bb, the seventh chord formed is a major seventh, which has a major triad as its
basis. This makes the root (Bb) extremely strong and clear, and the inescapable anchor of
the chord. With the B natural, the basis of the inverted seventh chord is a diminished
triad, which has no stability or sense of tonicity of its own. So the B natural exerts
much less power over the chord. If you then put it in a firmly D Minory context, say as
the tonic chord of a piece in D Dorian, the power of the D in the bass will easily be
enough to exert far mor strength than the B natural and emphasise the D minor triad as the
essence of the chord, to which the B is just an added "colour".

This makes it
worth giving it its own separate identity and chord symbol.

All of these 6ths are just add a mildly dissonant note to what are functionally triads -
so they are a flavour rather than a functional extension (unless you consider that for
instance a dm6 is just a 2nd inversion Bm7) If you're interested in functional 6th look up
the augmented 6th in its various flavours (French, Italian, and German) - you don't hear
that in many pop songs.

the "added sixth"
should rightly be seen as the 13th of the chord and if by choice you put it inside the
structure it's a closed voicing... IMO thinking of it as a sixth belies the nature of the
chord on a functional level... perhaps too subtly for some but there it is...

I'd have thought add6 means just a sixth added to a major or minor
triad, whilst 13 implies 11, 9 and 7 are also included in the chord. Different beasts
altogether.

Min add 6 is such a distinctive spicy chord that it tends (in my
rip off world of media music) to only get used for soundalikes of James Bond theme, 'that'
Guinness ad tune, Summertime and perhaps cheesy swing style Big Band sax quartets moving
between Maj add6 and dim7 chords.

Have to admit though, in the real world,
something like a dom 9th chord which actually has no 3rd eg. C,Bb,D,G (going upwards)
would often be referred to as a 9th chord in the context of say, a James Brownesque funk
guitar riff. Of course in theory world however, it snot though....

Just to add : In jazz Dm6 is used a lot as substitution for DmMaj7 (minor D scale
with a major seventh). I keep forgetting if that's harmonic or melodic minor, but it's a
minor that functions as the tonic, so it doesn't resolve to anything. By voicing a Dm6
instead of a DmMaj7 the chord sounds like it has "less attitude", more old school (think
1920's vs 1940's), but the tonic function remains the same. I allways voice that with the
9th instead of the root, so the voicing I use the most is : F A B E As for a Dmb6, I
have never seen that chord. It would indeed be written as a Bbmaj7/D, IMHO.

Another interesting way to voice Dm6 is in quartal voicings, with the root on top (on
piano). you then get B E A D.

Just my two pence added to the confusion

--------------------"Do not fear mistakes. There are none."
Miles Davis